Gram-positive bacteria are a remarkably diverse group of organisms that cause a variety of diseases in both humans and animals.
Some of the pathogens recognized as important in human and/or animal health include bacteria belonging to the families of Corynebacteriaceae, Enterococcacae, Micrococcaceae, Mycobacteriaceae, Nocardiaceae, and Peptococcaceae, which include such bacterial species as Actinomyces spp., Bifidobacterium spp., Corynebacterium spp., Enterococcus spp., Erysipelothrix spp., Eubacterium spp., Kytococcus spp., Lactobacillus spp., Micrococcus spp., Mobiluncus spp., Mycobacteria spp., Peptostreptococcus spp., Propionibacterium spp., and Staphylococcus spp. These pathogens cause a multitude of clinical manifestations in many different animal species. The treatment for such infections has historically been antibiotics that attack the common structures and functions of gram-positive organisms. However, many of the more ubiquitous gram-positive organisms have developed resistance to several classes of antibiotics, making treatment of infections difficult. The widespread use of antibiotics in the treatment of bacterial diseases in both humans and food production animals is likely a major contributing factor in the proliferation of antibiotic-resistant strains of many species of gram-positive organisms. Therefore, there is a great need to find different treatments that prevent or eliminate infections by gram-positive organisms in animals as well as humans.
Staphylococcal Infections in Agricultural Animals
In the agricultural industry a number of important diseases are caused by gram-positive organisms. Examples of clinical conditions caused by gram positive bacterial infections include, mastitis, septicemia, pneumonia, osteomyelitis, meningoencephalitis, lymphangitis, dermatitis, genital tract infections, metritis, perinatal disease, pituitary abscesses, arthritis, bursitis, orchitis, cystitis and pyelonephritis, caseous lymphadenitis, tuberculosis, ulcerative lymphangitis, erysipelas, laminitis, tyzzer's disease, tetanus, botulism, enteritis, malignant edema, braxy, bacillary hemoglobinuria, enterotoxemia. Staphylococcus spp., in particular, are capable of infecting many different species of agricultural animals and can cause enormous economic losses. For example, the United States dairy industry is estimated to lose approximately $185 per cow annually due to mastitis, a disease often caused by Staphylococcus aureus. Since there are 9.5 million head of milking cows in the U.S., the annual cost of mastitis is approximately $1.8 billion. This is approximately 10% of the total value of farm milk sales, and about two-thirds of this loss is due to reduced milk production in sub-clinically infected cows. Other losses are due to discarded abnormal milk and milk withheld from cows treated with antibiotic, costs of early replacement of affected cows, reduced sale value of culled cows, costs of drugs and veterinary services, and increased labor costs. In addition to its prevalence within the bovine dairy industry, mastitis caused by gram-positive cocci is also common among goats and sheep. Additional animal diseases caused by S. aureus include botryomycosis in horses, purulent synovitis and osteomyelitis in poultry, snuffles in rabbits, abortions in swine, and tick pyemia in lambs. Other species of staphylococci are major skin pathogens of canine (S. intermedius) and swine (S. hycius). In poultry species, staphylococcal pathogens cause endorcarditis and septicemia.
Staphylococcal Infections in Humans
Staphylococcus spp. are also human pathogens causing a wide variety of infections. The species Staphylococcus aureus, a common colonizer of human mucosa and skin, is an opportunistic pathogen that can cause diverse human infections. For example, S. aureus is the causative agent of several skin infections, including impetigo, furunculosis, cellulitus, and scalded skin syndrome, as well as potentially fatal post-surgical wound infections. In addition, the exposure of immunocompromised individuals to S. aureus in hospital settings has resulted in organ infections such as pneumonia, urinary tract infections, osteomyelitis, arthritis, bacteremia, and endocarditis. S. aureus is also the causative agent of toxinoses, most notably toxic shock syndrome and food poisoning. Food poisoning caused by the staphylococcal enterotoxin B is the most common cause of food-borne illness, surpassing even salmonellosis, campylobacteriosis and listeriosis. Other species of staphylococci also cause human disease; S. epidermidis, S. haemolyticus and S. hominis commonly infect implanted medical devices and S. saprophyticus is associated with urinary tract infections in women.
Virulence Mechanisms of Staphylococci
Staphylococci infect a variety of host tissues and evade the immune system through the production of several types of secreted proteins, surface expressed virulence factors and metabolic systems designed for survival amidst the limited resources and active defenses associated with the host environment. Colonization is the necessary first step in establishing infection; numerous factors including capsule, lipoteichoic acid, and teichoic acid are common structural components contributing to colonization. In addition, surface proteins such as staphylococcal fibronectin-binding protein and bone-sialoprotein binding proteins specifically bind host tissue components. Toxins are commonly produced among staphylococcal pathogens and are highly damaging; several human diseases, including food poisoning, toxic shock syndrome and exfoliative skin conditions, are the direct result of extracellular secreted toxin proteins. A single isolate may encode genes for 20-30 different secreted toxins. Some of the secreted protein products are superantigens that can bind nonspecifically to the MHC class II molecule of an antigen-presenting cell and, simultaneously, to the T-cell receptor of a T cell. The binding induces T cell signaling and leads to the release of high levels of proinflammatory factors, ultimately inducing host damage due to the overwhelming immune response. Another class of virulence factors expressed on the surface disguise the bacteria from the host immune system. For example, the S. aureus surface-expressed Protein A inhibits opsonization and phagocytosis by binding of the Fc component of host antibody. Numerous proteases, hemolysins (alpha, beta, gamma and delta), nucleases, lipases, hyaluronidase, and collagenase also aid bacteria in extracting nutrients from surrounding cells and protecting them against host defenses.
Antibiotic Resistance Among Staphylococci
The CDC estimates that each year nearly 2 million people in the United States acquire a nosocomial infection, resulting in 90,000 deaths annually. Of these fatal infections, 70% are caused by antibiotic-resistant bacteria. The increase in antibiotic-resistance among microbial species is particularly pronounced in skin and mucosal colonizers such as S. aureus. For example, the vast majority of S. aureus isolated from hospital settings are resistant to penicillin, and 50% are also resistant to the semisynthetic penicillins, such as methicillin, nafcillin, and oxacillin. These isolates, referred to as MRSA (methicillin resistant S. aureus) were first seen in the 1970s, and are now firmly established in hospital settings. Recently there have been several cases of MRSA infections in the community, where the infected individuals had no previous exposure to hospitals or healthcare workers. This alarming trend is intensified by the isolation of MRSA isolates that are less susceptible to vancomycin, a glycopeptide used to treat MRSA. Very few strains have been shown to be truly resistant to vancomycin according to the CDC's definition of vancomycin resistance, but several MRSA strains have been characterized as consisting of subpopulations with reduced susceptibility to vancomycin, or VISA (vancomycin intermediate S. aureus). Since the isolation of vancomycin resistant and vancomycin intermediate strains is a relatively new development, there is little data concerning their prevalence in hospitals and/or the community. Occasionally, VRSA (vancomycin resistant S. aureus) with full resistance to vancomycin and carrying a resistance plasmid likely acquired from Enterococcus spp. have also been recovered from humans.
Strategies for the Prevention and Treatment of Staphylococcus Infections
The emergence of numerous gram-positive pathogens that are resistant to multiple antibiotics has fueled research efforts aimed at developing preventative vaccines to protect against disease. Vaccines are designed to be administered to patients in order to elicit a long-term memory response from the immune system, so that if the pathogen is encountered at a future time, the immune system can more quickly and efficiently clear the pathogen. To date, a broadly-protective vaccine against gram-positive pathogens associated with a number of severe human diseases, particularly those disease associated with staphylococcal infections, is not available. Vaccine development approaches for the prevention of staphylococcal infections include those reporting the use of microbial surface components recognizing adhesion matrix molecules [MSCRAMMS (Nilsson et al. 1998. J Clin Invest 101:2640-9; Menzies et al. 2002. J Infect Dis 185:937-43; Fattom et al. 2004. Vaccine 22:880-7], surface polysaccharides (McKenney et al. 2000; McKenney et al. 1999. Science 284:1523-7; Maira-Litran et al. 2002. Infect Immun 70:4433-40; Maira-Litran et al. 2004. Vaccine 22:872-9; Maira-Litran et al. 2005. Infect Immun 73:6752-62) and mutated exoproteins (Lowell et al. 1996. Infect Immun 64:4686-93; Stiles et al. 2001. Infect Immun 69:2031-6; Gampfer et al. 2002. Vaccine 20:3675-84), as antigens in subunit vaccine compositions, as well as one live avirulent strain (Reinoso et al. 2002. Can J Vet Res 66:285-8) and several DNA vaccine approaches (Ohwada et al. 1999. J Antimicrob Chemother 44:767-74); Brouillette et al. 2002. Vaccine 20:2348-57; Senna et al. 2003. Vaccine 21:2661-6). Although many of these compositions have shown some degree of protection, they have achieved little cross-protection against diverse staphyloccocal strains and have additionally failed to elicit substantial immune responses in immunocompromised patients, an important at-risk population for nosocomial infections.
The most severe staphylococcal diseases are those mediated by the aforementioned supernantigenic pyrogenic exotoxins (SPEs) that nonspecifically stimulate T-cells independent of antigen presentation. Such diseases include toxic shock syndrome, exfoliative skin disease, and possibly Kawasaki syndrome. For these SPE-mediated diseases, immunotherapeutic agents that boost the immune system during an active infection are often more effective than vaccines, which are typically administered prior to infection. The overwhelming nature of the immune response to SPE necessitates rapid reduction in toxin activity as the first objective in therapy. To date, toxin neutralization in S. aureus-mediated disease has been most effectively accomplished by the administration of intravenous human immunoglobulin (IVIG), a purified, concentrated human antibody preparation from several thousand human donors (Takei et al. 1993. J Clin Invest 91:602-7; Stohl and Elliot. 1996. Clin Immunol Immunopathol 79:122-33). The widespread distribution of S. aureus, which colonizes approximately 30% of healthy human adults, coincides with high exposure rates for the majority of the population, so the level of anti-staphylococcal anti-toxin antibodies in WIG is often sufficient to neutralize toxin long enough to stabilize the immune response until the bacterial load is reduced with antibiotics (Schlievert, 2001. J Allergy Clin Immunol 108(4 Suppl):S107-110). IVIG preparations from multiple manufacturers have been shown to neutralize toxin in proliferation assays with human peripheral blood mononuclear cells, inhibit toxin-induced human T cell-driven B cell differentiation in vitro (Stohl and Elliot. 1996. Clin Immunol Immunopathol 79:122-33; Stohl and Elliott. 1995. J Immunol 155:1838-50; Stohl et al. 1994. J Immunol 153:117-27) and reduce IL-4 and IL-2 secretion in PBMCs stimulated with staphylococcal enterotoxin B (Takei et al. 1993. J Clin Invest 91:602-7; Darenberg et al. 2004. Clin Infect Dis 38:836-42). IVIG therapy, with its proven ability to neutralize SPE, is now a recommended therapy for Kawasaki syndrome and is gaining favor as a treatment method for staphylococcal toxic shock syndrome (Schlievert 2001. J Allergy Clin Immunol 108(4 Suppl):S107-110). Use of IVIG as an immunoprotective wound lavage during surgery has also been investigated in mice (Poelstra et al. 2000. Tissue Eng 6(4):401-411). Although standard IVIG has utility for limiting the advance of some staphylococcal SPE-mediated disease, the safety, efficacy and consistency of human IVIG preparations generated from thousands of unselected human donors remains controversial (Baker et al. 1992. N Engl J Med 327:213-9; Miller et al. 2001. J Allergy Clin Immunol 108:S91-4; Sacher, 2001. J Allergy Clin Immunol 108:S139-46; Darenberg et al. 2004. Clin Infect Dis 38:836-42). Furthermore, the benefit of IVIG in preventing some staphylococcal infections is doubtful (Baker et al. 1992. N Engl J Med 327:213-9; Hill, H. R. 2000. J Pediatr 137:595-7; Darenberg et al. 2004. Clin Infect Dis 38:836-42). In order to increase the effectiveness of IVIG in treating staphylococcal infections in certain at-risk populations, a plasma-derived, donor-selected, polyclonal anti-staphylococcal human IgG with high titers of antibody directed toward the staphylococcal MSCRAMMS clumping factor A (ClfA) and fibrinogen-binding protein G (SdrG) was created and tested with success in very low birthweight infants to prevent staphylococcal sepsis (Vernachio et al. 2003. Antimicrob Agents Chemother 47:3400-6; Bloom et al. 2005. Pediatr Infect Dis J 24:858-866; Capparelli et al. 2005. Antimicrob Agents Chemother 49:4121-7). A specific humanized monoclonal antibody toward the S. aureus MSCRAMM Clumping factor A, is also being developed. The antibody was selected from a pool of thousands of murine anti-ClfA antibodies for its ability to bind ClfA in a manner that abrogates S. aureus binding to human fibronectin and was subsequently humanized by mutating specific targeted residues to mimic the homologous human germline subgroup antibody (Hall et al. 2003. Infect Immun 71:6864-70; Domanski et al. 2005. Infect Immun 73:5229-32). The specific antibody is being designed for use in conjunction with antibiotics for the treatment of severe life-threatening S. aureus infection, although animal studies also demonstrated a prophylactic protective effect.